[Wasm Ryujit] Wasm SP is implicitly referenced

[AndyAyersMS]

Fixes an issue where SP mentions are introduced late (say from a throw helper).

[AndyAyersMS]

@SingleAccretion not sure if this is what we agreed on or you have other ideas.

Fixes things like

Assertion failed '(WasmValueType::Invalid < wasmType) && (wasmType < WasmValueType::Count)' 
   in 'System.Reflection.BlobUtilities:WriteByte(byte[],int,byte)' during 'Generate code'

where the only SP mention is in the bounds check helper.

[AndyAyersMS]

fyi @dotnet/jit-contrib

[Copilot]

Pull request overview

This PR ensures the Wasm shadow stack pointer local (lvaWasmSpArg) is treated as “implicitly referenced” so it won’t be considered unused when references to it are introduced later in compilation (e.g., via late-added helper calls).

Changes:

• Mark the Wasm stack pointer argument local as lvImplicitlyReferenced.
• Mark the reverse-P/Invoke Wasm stack pointer temp local as lvImplicitlyReferenced when it is allocated.

[SingleAccretion]

I think the ordering issue between the stack level setter and liveness / ref counting still needs to be fixed, but this is also a correct change (since there are implicit references at nodes like LCLHEAP).

[AndyAyersMS]

@adamperlin @kg can one of you approve?

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